A conventional mineral process technique for separating sulphide minerals from ores rich in magnesium minerals involves the following steps:                (i) crushing and wet milling of the nickel sulphide ore to form a pulp having particles of a desired particle size distribution;        (ii) adding frother, collector and depressant to the pulp;        (iii) adding acid to the pulp;        (iv) adding an activator to the pulp;        (v) floating the valuable minerals in a rougher-scavenger stage with the primary object of maximising the recovery of the valuable sulphide minerals, and        (vi) refloating the froth product from the rougher-scavenger stage in a cleaning stage with the object of producing a concentrate of the required quality by rejecting a maximum amount of gangue minerals and a minimum amount of valuable minerals.        
The addition of collector makes the sulphide minerals hydrophobic and the addition of depressant minimises the recovery of gangue minerals to the flotation concentrate. The addition of acid and activator enhances the effect of the collector and, in turn, improves either recovery or grade or both. The flotation concentrate of valuable sulphide minerals is filtered and dried in preparation for smelting, or other secondary treatment processes such as leaching. For smelting or for other secondary processing, the amount of gangue, particularly magnesium bearing gangue, should be minimised.
It is recognised that small additions of reagents in the cleaning stage can improve the flotation of valuable sulphide minerals and can reduce the recovery of gangue. For the flotation of nickel ores rich in magnesium bearing minerals such reagents can include acid or base to lower or raise the pH, copper sulphate to activate the sulphides and polysaccharides to depress the flotation of the gangue minerals. It is also recognised that small additions of collector and frother throughout the circuit can be beneficial. Unfortunately, for many magnesium bearing ores, the addition of acid or base is poorly effective. For example, the addition of acid can promote the flotation of the valuable minerals but, in turn, cause low grade composite particles to float into the concentrate and lower the grade. Conversely, the addition of base can depress the flotation of the composite particles and, in turn, raise the concentrate grade, but the recovery is then reduced because the composite particles, and sometimes some liberated valuable particles, are lost from the froth phase. This problem can be particularly severe for nickel ores containing large amounts of magnesium bearing minerals.
A number of strategies have been employed in an attempt to overcome the competing effects of acids and alkalis and of activators and depressants in cleaner flotation circuits, these strategies including:                (i) making small staged additions of different reagents at various points in the circuits, and        (ii) floating at a pH value that is intermediate between that for strong flotation of liberated particles and that for weak flotation of composite particles.        
These strategies tend to be relatively ineffective and their applications are restricted and/or the benefits are limited, for example, in the cleaning circuit at the Mt Keith, Western Australia, concentrator of WMC Resources, only small additions of acid or activator can be made before large amounts of low grade composites are floated into the concentrate and the grade of the final product becomes unacceptably low. This is particularly a problem with low grade nickel sulphide ores, high in magnesium bearing minerals such as the ore treated at Mt Keith, Western Australia.